1. Field of the Disclosure
The present disclosure relates to a vertical external cavity surface emitting laser (VECSEL), and more particularly, to a VECSEL in which the efficiency of a second harmonic generation (SHG) crystal is improved by reducing a full-width at half maximum (FWHM) of laser light using two etalon filters.
2. Description of the Related Art
A vertical external cavity surface emitting laser (VECSEL) is a laser device having a high output of several to several tens of Watts by replacing an upper mirror of a vertical cavity surface emitting laser (VCSEL) with an external mirror and increasing a gain region.
FIG. 1 is a schematic view of a conventional VECSEL 10. Referring to FIG. 1, the VECSEL 10 includes a laser chip 18 including a Distributed Bragg Reflector (DBR) layer 11 and an active layer 12, a heat spreader 13 for cooling the laser chip 18, a first mirror 15 separated from the laser chip 18, and a second mirror 17 reflecting laser light reflected from the first mirror 15 back to the first mirror 15. The VECSEL 10 further includes a second harmonic generation (SHG) crystal 16 doubling the frequency of the light along an optical path between the second mirror 17 and the first mirror 15. A birefringence filter 14 is disposed along an optical path between the first mirror 15 and the laser chip 18. As is well known in the art, the active layer 12 may have a multiple quantum well structure having a resonant periodic gain (RPG) structure, and is excited by a pumping beam to emit light at a predetermined wavelength.
In the above described configuration, when a pumping beam emitted from a pump laser (not shown) is incident on the active layer 12, the active layer 12 is excited and emits light at a predetermined wavelength. As illustrated in FIG. 1, the VECSEL 10 may have a structure in which a pumping beam is incident on a bottom surface of the active layer 12 or a structure in which a pumping beam is obliquely incident on a top surface of the active layer 12. The laser light generated in the active layer 12 is reflected by the DBR layer 11 to the first mirror 15 and is reflected again by the first mirror 15 to the second mirror 17. Thus the wavelength of the laser light is reduced by half by the SHG crystal 16. For example, if the laser light generated in the active layer 12 is infrared light having a main wavelength of 920 nm, the light passing through the SHG crystal 16 becomes visible light having a main wavelength of 460 nm.
The second mirror 17 has high reflectivity with respect to the visible light, and may be coated to slightly transmit the infrared light whose wavelength is not converted. Accordingly, the light which is converted by the SHG crystal 16 is reflected by the second mirror 17, and is output to the outside through the first mirror 15. The infrared light whose wavelength is not converted can be output to the outside through the second mirror 17. The birefringence filter 14 filters laser light and allows laser light at a predetermined wavelength to resonate. Furthermore, the heat spreader 13 exhausts heat generated in the active layer 12 to cool the active layer 12.
The SHG crystal 16, as illustrated in FIG. 2, has high wavelength conversion efficiency at a very narrow wavelength band. That is, the SHG crystal 16 has wavelength conversion characteristics of a very narrow full-width at half maximum (FWHM). For example, when the SHG crystal 16 is periodically poled stoichiometric lithium tantalate (PPSLT), the FWHM is about 0.1-0.2 nm. However, the FWHM of the laser light in the infrared light range which is output by the second mirror 17 is relatively large, and thus the conversion efficiency of the SHG crystal 16 is lowered. For example, without the birefringence filter 14 and the heat spreader 13, the FWHM of the output laser light is about 1.6 nm, and thus most of the laser light is not wavelength-converted and wasted.
The FWHM of the output laser light can be reduced to some extent by the birefringence filter 14 and the heat spreader 13. Generally, the FWHM of the laser light is decreased when the thicknesses of the birefringence filter 14 and the heat spreader 13 are increased. For example, when the thickness of the heat spreader 13 is 300 μm and the thickness of the birefringence filter 14 is 4 mm, the laser light has a FWHM of 0.29 nm at a main wavelength of 920 nm and a FWHM of 0.35 nm at a main wavelength of 1064 nm. When the thickness of the heat spreader 13 is 500 μm and the thickness of the birefringence filter 14 is 4 mm, the laser light has a FWHM of 0.26 nm at a main wavelength of 920 nm and a FWHM of 0.3 nm at a main wavelength of 1064 nm. When the thickness of the heat spreader 13 is 500 μm and the thickness of the birefringence filter 14 is 6 mm, the laser light has a FWHM of 0.26 nm at a main wavelength of 920 nm and a FWHM of 0.27 nm at a main wavelength of 1064 nm. However, to have a sufficiently small FWHM, the thicknesses of the birefringence filter 14 and the heat spreader 13 must be very large, and thus the manufacturing costs increase and the size of the VECSEL increases as well. Moreover, when the thicknesses of the birefringence filter 14 and the heat spreader 13 increase, the output power of the laser light decreases. Therefore, it is difficult and impractical to reduce the FWHM of laser light by increasing the thicknesses of the birefringence filter 14 and the heat spreader 13.